Most of the small grounding systems of power grid are neutral ungrounded systems, and the change of single-phase grounding protection has developed from traditional grounding protection to unattended substation with integrated automation device grounding protection, grounding line selection device, etc., its protection mainly includes the following:
(1) System grounding insulation monitoring device:
The insulation monitoring device uses the zero sequence voltage to monitor the ungrounded system.
One of the windings of the bus voltage transformer in the substation is connected into a star shape, and the voltmeter is used to monitor the relative ground voltage. The other winding is connected into an open triangle, and the overvoltage relay is connected to reflect the zero sequence voltage when the ground fault occurs.
When the occurrence of single-phase grounding fault, the opening triangle appears zero sequence voltage, overvoltage relay action, grounding signal.
The protection can only detect the grounding fault, and distinguish the grounding phase through three voltmeters, but can not distinguish which line is grounded. In order to judge the fault line, it must be tested by the pull line, which will increase the number of power outages for users. And if more than two lines of ground fault occurs, it will be more difficult to distinguish.
The device may mistakenly send or reject grounding signal due to ferromagnetic resonance of voltage transformer, bad contact of fuse, dc grounding and bad contact of loop. (2) Zero-sequence current protection: zero-sequence current protection is to use the zero-sequence current of fault line is larger than the zero-sequence current of non-fault line to achieve selective protection, such as DD-11 grounding current relay and RCS-955 series of nanzi factory protection.
The protection is generally installed on the line of the zero-sequence current transformer, and its sensitivity and selectivity can be guaranteed in the power grid with more outgoing lines. However, due to the error of the zero-sequence current transformer, the complexity of the line connection, the size of the single-phase grounding capacitor, the error of the device, the error of the fixed value, the leakage current of the conductive sheath of the cable and other influences, the zero-sequence current secondary reflection of the single-phase grounding fault line is not necessarily larger than that of the non-fault line, which is prone to misjudgment and misoperation.
(3) Zero sequence power protection:
Zero-sequence power directional protection is a selective protection based on the zero-sequence current difference of 180° between non-fault line and fault line. Such as the traditional zero sequence power direction relay, unattended integrated from the application of such as Nanrui DSA113, 119 series zero sequence power direction protection.
The zero-sequence power direction protection has no dead zone, but it has high requirements on the zero-sequence voltage and zero-sequence current circuit wiring, and the fifth harmonic power principle is needed for the arc-suppression coil in the system.
(4) Small current grounding line selection integrated device:
With the development of electric power technology, independent small grounding current line selection device has been gradually applied in small current grounding power system in recent years. Grounding judgment and line selection for all outgoing leads of small current system, such as MLX series of Huaxing Company. MLX series line selection device principle is to use current (arc suppression coil grounding with five harmonics) line direction judgment, the largest of the three selected current line in the direction of comparison, so as to solve the zero sequence current is small, all kinds of LH error, measuring error, power cables, the undercurrent and arc suppression coil, the capacitor charge and discharge process, correctly judging or removal of fault lines.
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